Blocking tube oscillator for television



2 Sheets-Sheet 2 Filed July 15, 1941 mo M ZNVENTOR.

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- BLOCKING TUBE OSCILLATOR FOR TELEVISION Richard L. Campbell, Hasbronck Heights, N. 1., assignor to Allen B. Du Mont Laboratories, Inc., Passaic, N. 1., a corporation of Delaware Application July 15, 1941, Serial No. 402,498

A Claims.

This invention relates to a blocking tube oscillator that is particularly useful in television circuits although it is not restricted to this particular use. While blocking tube oscillators have been known for a long time, by the present invention improvements are made in the blocking tube which result in more stable performance and in ability to reject interfering signals more completely. Blocking tube oscillators are used, for example, in television receivers to generate control pulses for producing sawtooth wave forms for scanning. In some cases the blocking tube oscillator delivers only an impulse at scanning frequency and in other cases the blocking tube oscillator circuit also includes components to cause the actual sawtooth wave form'to be generated directly.

Blocking tube oscillators are generally synchronized or controlled in their trigger oil periods by means of incoming synchronizing signals. It is very desirable that a blocking tube oscillator be selectively responsive to the synchronizing rier, pulses of which are used for synchronizing purposes. In this way only those synchronizing signals which have th frequency of this car- I rier are passed by the transformer and furthermore this same frequency of resonance in the transformer is used as the oscillation frequency for the blocking action.

This invention will be specifically described as being suitable for using carrier synchronizing pulses for vertical scanning frequency similar to those described in my Patents Nos. 2,249,942 and 2,249,943. However, the invention is also suitable for other carrier synchronizing pulses such as the horizontal channel, for example, where the carrier frequency may be of the order of 3 megacycles, more or less.

My Patent 2,249,943 shows one way of utilizing a carrier signal at a receiver employing a thyratron type of discharge tube. There are other pulses while at the same time it rejects other types of signals which'are present in the incoming waves. Earlier forms of blocking tube oscillators utiiized an iron-cored transformer for feeding energy back from the plate circuitto the grid circuit of the blocking tube. The performance of such circuits depends upon over coupling so that the tube tends to oscillate at a frequency determined by the constant of the iron-cored transformer and the grid circuit is provided with the condenser and resistance network. so that the tube builds up a bias voltage at the initial oscillation suflicient to cut the tube of! for an interval. This condenser and resistance network slowly discharges to the point where the tube again tends to oscillate until another blocking cycle occurs. In the prior devices the frequency at which the circuit would tend to oscillatewas determined by the iron-cored transformer and bore no particular relation to the synchronizing signal. In those circuits the synchronizing signal generally took the form of a square wave impulse of short duration which .might be coupled to the blocking tube transformer by use of a third winding for example. However, such a circuit was susceptible to extraneous noise signals which simulated the synchronizing pulses.

These and other diiflculties areovercome by the present invention. In carrying out this invention the iron-cored transformer is replaced by a radio frequency transformer at least one winding of which is tuned to the frequency of a carways of using a carrier synchronizing pulse with a blocking tube oscillator but the particular way utilized in the present invention combines the blocking tube action with the synchronizing selective action in one simple radio frequency transformer.

The invention may be understood from the 7 description in connection with the accompanying back transformer;

drawings in whichi Fig. l is a schematic drawing showing a blocking tube oscillator using a radio frequency feed Fig. 2 represents the wave of carrier for synchronizing;

Fig. 3 represents the wave form of the voltage on the grid of the blocking tube oscillator;

Fig. 4 represents the corresponding wave form of the plate current in th blocking oscillator tube;

Fig. 5 represents the output sawtooth wave form; and r Fig. 6 is a view similar to Fig. 1, showing a modification. In the drawings, reference character I indicates a lead which introduces the synchronizing signals for a television receiver and may also include picture components which are to be removed subsequently in the known ways by amplitude selection. Condenser 2 connects these signals to the grid 3 of the tube 4. Resistor 5 maintains the bias on grid 3. The plate 8 carries only the synchronizing signals since tube 4 performs a synchronizing separator function. These synchronizing signals are fed to the winding 1 of the transformer I which has two unform of a few bursts. Q

maybe tuned by condenser I I so that it resonates at the frequency of the "incoming synchronizing pulse. of carrier. These carrier pulses are applied through the secondary I of transformer 8 to the grid I2 of tube I3 which i the blocking oscillator tube. The plate I4 of this tube is connected to winding 9 of the transformer 8 with such polarity as to feed energy back into the grid circuit by means of windin I II. In this 'way the tube I3 in conjunction with the transformer 8 'forms an oscillator. Insertion of resistor I5 shunted by condenser I6 in the grid return lead ll of winding I0 provides a means whereby oscillations which take place in tube I3 are caused to charge up condenser IE to. a point which will bias the grid I2 to cut off with respect to the cathode I8. The tube I3 then becomes inactive for an interval until the charge on condenser I5 has leakedofi through the variable resistor I5 sufliciently to allow the oscillations to resume in tube I3, starting a new blocking cycle. This pulsating flow of current in the plate circuit of tube I3 is used as a discharge mechanism across the condenser 20 which is connected by lead 2| .to one end of winding 9 the other end of which is connected by lead 22 to the plate I4.

The fixed resistor 23 and adjustable resistor 24 in series serve to charge condenser 20 from the B+ supply during the blocked interval of tube I3. In this way conductor 25 is caused to carry a sawtooth wave form which may be applied to deflection plates of a cathode-ray tube :in the conventional way for sweep purposes, either for television scanning or oscillographs or the like.

Fig. 2 shows somewhat diagrammatically a wave form representing bursts of carrier energy lasting for short intervals with longer intervals between bursts free of signal, which may be applied through the lead I. The carrier waves are of high frequency but the repetition of the bursts 30 are of much lower frequency. For example, these bursts may be either at the scanning rate for television lines or television fields. A 500 kilocycle carrier wave at a pulse repetition rate of 60 per second for vertical scanning is described in my Patent 2,249,942. Also a carrier of 3 megacycles with pulse repetition rate of approximately 15,000 per second, for example, might be used for horizontal synchronization. In Fig. 2 reference character 30 represents a few oscillations of each burst of the synchronizing carrier and M represents the dormant interval in which no signals of this frequency are present.

The wave form '34 in Fig. 3 represents the voltage at the grid I2 of tube I3. The dotted line 35 represents the critical voltage level at which tube I3 begins to conduct current. The tube I3 has a spontaneous blocking action in which the dotted wave form 36 would be followed except for the synchronizing information or impulses 30. At the point 31 the voltage of grid I2 would cause a condition so that plate current would begin to flow. The windings 9 and II) are so connected that this increase of plate current would induce a further positive voltage on grid I2, causing grid current to flow since grid I2 becomes positive v with respect to' cathode I8. ,However, the presence of the synchronizing pulse 30 causes the grid I2 to reach the critical triggering voltage at the point 38 in the wave form instead of the point 31, thus drawing the spontaneous period of the blocking oscillator in step with the synchronous period-of the incoming synchronizing lation to cease and leaves a charge on condenser 6 so that grid I2 is held considerably beyond cut-off for an interval. The resistive leak I5, however, gradually removes the charge on condenser I6 until the grid I2 again starts a new blocking cycle. In the regions from about 39 through 40 to about 4| the blocking oscillator is relatively unresponsive to incoming signals of either synchronizing pulses or extraneous noise.

In the region from about 4| to 42 along the wave form the oscillator is readily susceptible to lock-in by incoming synchronizing pulses and is also susceptible to noise However, by the use of the tuned transformer 8 the circuit is highly selective to accept the desired synchronizing carrier and to reject extraneous noise even in this interval. In the region from 42 to 43' the tube I3 is conducting and therefore the winding I0. heretofore highly resonant, is now heavily damped by the conducting grid I2 so that once the blocking tube action is triggered off at 38 by a burst 30 of incoming synchronizing carrier the synchronizing channel is effectively disconnected for further-impulses either of synchronizing or noise nature until the next sensitive period of the blocking action is reached. The region 42 to 43 is shown on the wave form I 44 as an envelope or low frequency component of the probable waves on grid I2. The voltage on grid i2 probably takes the form of an oscillatory wave in the region 42 to 43 having the natural period of the tuned circuit determined by trans-. former winding II! and the tuning condenser II modified by the parallel circuit components. In prior devices this natural period of oscillator which may introduce a fine structure wave form in the region from 42 to 43 was determined by an iron-cored transformer and had no particular relation to the incoming synchronizing pulse carrier frequency.

The wave form 44' in Fig. t shows the current with peaks 45 along the lead 2| and Fig. 5 shows the sawtooth wave form of voltage 46 resulting from the capacity 20 and resistances 23 and 25 which wave form passes along the lead 25 to the deflecting plates.

Fig. 6 shows another embodiment of the inone embodiment of the invention are given.

Tests have shown the superiority of this type of blocking tube oscillator with radio frequency synchronizing signals over the prior blocking tube oscillators using the square wave type of synchronizing signals.

In Fig. 6 the blocking oscillator transformer 5| has two windings 52 and 53 connected in series, the winding 52 being left untuned while the winding 53 is shown as being tuned to 530 kilocycles by means of the condenser 54. The double triode tube 55 which in this embodiment is a 6C8G type has its first section connected as a blocking oscillator in which the grid 51 receives its feed-back energy from the winding 53 through the coupling condenser 58. This condenser 58 receives a resistor ligand the adJustable resistor l2-leak the charge on of the condenser II so as to allow: the

first triode section of tube I! to conduct current again. The-plate ll of this section provides the blockingoscillator feed-back to the winding I2 01' Q [the transformer II. The coupling between windings 52 and II is great enough to cause the blocking oscillator action as explained above in con- '1 1 Templej the . Whenreslstor is made variablejthe' amplitude of sawtootliyoltage built up across con- ,densei' I is. thereby; made controllable without as the period of the blocking oscillator cycle would be affected in- Fig. 1 when the amplitude nection with Fig. 1. The power supply voltage for the plate 04 is applied through the winding 02- b meansof the conductor 65. The resistor l0 "andcondenser I! serve as a de-coupling filter toreduce radiation from this circuit and to precircuitsconnected to the power supply. A low resistance to ground is inserted into the cir-' cult-of the cathode N. This resistance III acts .Ivent spurious pickupin this circuit'from other jointly as a load circuit to deliver signal from 1 actsas an .input resistor for the synchronizing signals which are injected into the blocking oscillator through the cathode. The synchronizing signals, which may, for example, be bursts of 530 kilocycle waves, are first separated from the picture components andhorizontal synchronizing components in well known ways by circuits not shown.- Means for obtaining the separated 530 kilocycle carrierbursts for verticalsynchronizing have been described in my Patent 2,249,943, supra. These synchronizing bursts of carrier frequency are applied to the cathod 8! by means of the small coupling condenser 12, the resistance ll being part of the output circuit from the previous separating circuits. The conductor I4 leads an output impulse from the cathode II to the grid I5 of the next triode section of the tube II. The signal on the conductor 14 is a short burst of energy somewhat similar to that shown in Fig. 4. This burst of voltage corresponds to the current passed from plate 6| to cathode O0 in the first section of the tube during the conducting portion of the blocking tube oscillator cycle since this signal on conductor 14 contains the residual of the synchronizing signal'coupled through condenser! and contains also the oscillatory current of the blocking oscillator action. It is desirable to-pass the signal through the integra'ting network consisting of resistor'" and condenser ll thus arriving at an envelope volta e on the conductor 19 which is similar to that illustrated in Fig. 4. n The resistor 8| 'is inserted between the conductor .19 and the grid for further filtering and to allow self-bias on the grid 15. In this way the second triode section.

consisting of grid I5, cathode l2 and plate 84 can generate a sawtoothvoltage whose period is controlled by the repetition rate of the blocking oscillator impulses provided from the cathode ll of the first triode section and filteredas described above. The condenser 85 is connected to the plate 84. It is charged through the resistor during the time when no impulse is applied to grid 15. The condenser 85 is then discharged by current flowing from plate 84 through cathode I! during the time that the impulse is applied to grid I5. In this way a sawtooth voltage is delivered through the condenser 88 to the conductor 89 which leads to the conventional amplifier equipment for applicationto a cathode-ray tube.

The circuit of Fig. 6 provides more freedom'of amplitude control forthe output sawtooth volt- V the first triode section of the tube 55 and also age than the somewhat simpler circuit of Fig. 1.

affecting the period jofthe blocking oscillator cycle in the first-triode section nearly so much control 24' is adjusted. However, both Fig. 1 and Fig. 6 show embodiments of this invention which employ 'ablock-ing tube oscillator coupling transi former whose resonant frequency is the same as the frequency of the carrier waves in the bursts of synchronizin'gpulses. While the particular circuit-constants given in '6' are active at 530 kilocycles as a. carrier frequency for vertical synchronization, it is to be understood that such a circuit usingthese principles can also be adapted to operate at other' carrier frequencies to ,provide superior synchronization and .sawtooth generation for television scanning. For example a 3 niegacycle carrier burst at line periodicity of approximately 15,000 bursts' per second has proved to be very satisfactory. I

Some or the advantages of this invention are that there is amore precise or accurate triggering of the oscillator; there is a higher signal to noise ratio; than is. usual in prior. devices; there is a positive control of the oscillator in which the frequency of the control is high and the periodicity is low; the periodicity of the control information is utilized to control the oscillator periodicity; and satisfactory synchronization and stable interlock is secure. The invention is suitable for controlling either horizontal synchronization or vertical synchronization for-television systems as well as for controlling other devices.

What is claimed is:

1. In combination, a blocking tube oscillator suitable for generatingco'ntrol. pulses for sawtooth waves, and means for applying bursts of carrier frequency energy to said oscillator to trigger the same. I 2 In combination, a, blocking tube oscillator suitable for generating control pulses for sawtooth waves, and means for applying bursts of carrier frequency energy to said oscillator to trigger'the same, said'oscillator comprising an air core transformer.

3. In combination, a blocking tube oscillator, suitable for generating control pulses forsawtooth waves, and means for applying bursts of carrier frequency energy to said oscillator to trigger the same, said oscillator comprising a radio frequency transformer- 4.In combination, a blocking tube oscillator suitable for generating control pulsesfor sawtooth waves, and means for applying bursts of carrier frequency energy to said oscillator to trigger the same, and means connected to the output of said oscillator to produce a sawtooth wave of energy.

5. In combination, a blocking tube oscillator, suitable for generating control pulses for sawtooth waves, and .means for applying bursts of carrier frequency energy to said oscillator to trigger the same and having its natural period of blocking slightly greater than the period of said bursts of energy.

6. In combination, a blocking tube oscillator.

suitable forgenerating control pulses for sawtooth waves, and means for applying bursts of carrier frequency energy to one of the electrodes of said oscillator to trigger said oscillator.

7. In combination, a blocking tube oscillator suitable for generating control pulses for sawtooth waves, and means for applying bursts ofquency energy to said oscillator to control its periodicity. v

9. A blocking tube oscillator for generating, control pulses for sawtooth waves having a coupling network for feed back comprising a winding and means for tuning it so that it is resonant to television synchronizing information of carrier frequency and means for applying to said winding bursts of carrier frequency energy. a l

10. In combination, a blocking tube oscillator having a coupling network for feed back that is resonant to a desired carrier frequency and means for applying synchronizing pulses of carrier frequency to said network.

11. In combination, a blockingtube oscillator having a coupling network for feed back that is resonant to a desired carrier frequency, and means for applying synchronizing pulses of carrier frequency to said network, said network comprising anlair core transformer.

12. In combination; a blocking tube oscillator having a coupling network for feed back that is resonant to a desired carrier frequency. and means for applying synchronizing pulses of carrier frequency to said network, said network means for applying synchronizing pulses of carrierfrequency to said network, said network comprising an air core transformer, the freqency of resonance in said transformer being suitable for use as the oscillation frequency for the blocking action of said tube.

14. In combination, a blocking tube oscillator having a coupling network for feed back that is resonant to television synchronizing information-'of carrier frequency, and means for applying" bursts vof carrier frequency energy ,to said tube.

15'. In combination, a blocking tube oscillator having a coupling network for feed back that is:

- RICHARD L. eamnnili 

